Engineering and Design Tools - TSOC 302 and TSOC 303 - 2 Design for Manufacture

2 Design for Manufacture

2.1 Introduction Design for manufacture or 'Manufacturability' concerns the cost and difficulty of making the product. At a simple level manufacturability, design for manufacture (DFM) at a part level, involves detail such as ensuring that where a pin is to be assembled into a hole that is only slightly larger in diameter, then it is much easier if the end of the pin or the entry to the hole (or both) are chamfered or finished with a radius. This applies whether the assembly is carried out manually or automatically. This is a fine tuning process carried out once the product form has been decided. Indeed automatic assembly would be very difficult / expensive if neither component of a close fitting pair was chamfered. At a more complex level, product DFM tackles the more fundamental problem of deciding on the product structure and form. Design for assembly (DFA) is an important part of this.
Some 'manufacturability' software is available, relating both to manufacture and to assembly.
This section starts with some simple but important principles of manufacturability.

2.2 general Principles of manufacturability

1. Reduce the number of parts by combing function, eg snap fasteners
2. Use a robust design
3. Use modular design
4. Design so assembly operations are in 1 direction
5. Design so it is only possible to assemble the components in the correct way
6. Use popular standard / preferred sizes
7. Minimise the use of fasteners
8. Eliminate or simplify adjustments
9. Avoid flexible components
10. Dimensions in each direction should be from 1 datum
11. Avoid sharp corners, use generous fillets and radii
12. Use simultaneous (or concurrent) engineering
13. Do not specify tolerances tighter than necessary
14. Do not specify surface roughness smoother than necessary.

2.2.1 Reducing the number of parts frequently reduces the weight of the product which is advantageous. Eliminating the need for a separate housing or enclosure can be beneficial. One method that has been successful in many cases is to replace a fabricated sub - assembly, which may utilize many fasteners, with a single casting (ref. M2). In some cases this has given weight savings as well as cost savings.

2.2.2 A robust design is one that has been optimised so that variations from the nominal specification cause a minimum loss of quality. To determine these optimal values will normally necessitate experimental work on a prototype (ref. M1).

2.2.3 The assembly of products made up from 4 to 8 modules with 4 to 12 parts per module can usually be automated most readily. It is also helpful to maintain a generic configuration as far as possible into the assembly process and install specialist modules as late as possible.

2.2.5 Designing so only correct assembly is possible is useful where semi - skilled labour is used and it is also desirable if there are safety considerations if the product were to be incorrectly assembled. Manufacturers of mains powered consumer electrical appliances frequently supply them with a flex having a moulded on supply plug. This minimises the danger of the consumer incorrectly wiring a plug and suffering an electric shock.

2.2.6 Using standard sizes will reduce costs directly and reduced delivery times will indirectly give savings. This will also reduce the cost of repairs and maintenance.

2.2.7 Fasteners can add significantly to costs, frequently the cost of installation will greatly exceed purchase cost. If fasteners must be used then minimise the sizes and types. Small fasteners and parts should be avoided.

2.2.8 Mechanical adjustments add to the cost of fabrication and cause assembly, test and reliability problems. The need for adjustments can often be negated by using dowel pins, detents, notches or spring mounted components. If a designer understands why an adjustment has been recommended, a way of eliminating or reducing the need can often be found.

2.2.9 Wiring and other flexible components are difficult to handle during assembly. The use of rigid or process applied gaskets, circuit boards rather than electric wiring helps to minimise this problem.

2.2.10 Dimensioning from 1 datum simplifies gauging and minimises errors in tolerances. Dimensions should also be measured from points or surfaces on a component, not points in space.

2.2.11 Using large radii is generally good practice for most processes, casting, forming etc. as material flow is facilitated - and stress concentration is reduced. However sharp corners are inevitable with some processes, eg 2 intersecting machined surfaces and punch face - wall edge in a powdered metal component. There is no cost advantage in preventing these sharp corners.

2.2.12 In simultaneous, or concurrent engineering, personnel from functions other than design are involved in the design process, including manufacturing specialists. This enables all aspects of a design to be considered at an early stage.

2.2.13 This can be critical, particularly for closer tolerance parts because as tolerances become tighter, the rise in manufacturing costs is increasingly steep.

2.2.14 As for 2.2.13, The costs of generating smoother surfaces rise steeply.

Specific recommendations for materials and processes can be found in ref. M1.